As long ago as 1948, a demonstration plant for the gasification of coal in suspension was built and operated at the U.S. Bureau of Mines facility in Missouri. Subsequently, the first commercial plant that used the Koppers-Totzek coal gasification process was built and operated in Finland, and since then additional commercial coal gasification plants using the Koppers-Totzek process have been built and are operating in various countries of the world. The Koppers-Totzek process is recognized and understood by those skilled in the art to be a process for the gasification of coal in suspension. Hereinafter the Koppers-Totzek process is designated as KTP.
In order to avoid the fouling of heat transfer surfaces of the waste heat boilers used in the KTP, it is necessary to solidify the liquid slag droplets that are entrained in the gas leaving the gasifier, and to cool the liquid slag droplets to a temperature at which they are not tacky. This means that the entire gas stream leaving the gasifier must be cooled to a temperature that is about 100.degree.F (38.degree.C) below the slag softening temperature. For most coals, the softening temperature of the ash is in the range of about 1900.degree.F to 2400.degree.F (1037.degree.C to 1316.degree.C). In the KTP it is customary to operate the gasifier at a temperature of about 2700.degree.F (1482.degree.C) and to use a water spray to quench the hot gas just as it leaves the gasifier, before it enters the waste heat boiler.
In the current method of operating a KTP facility, the water droplets from the sprays are entrained in the high velocity gas leaving the gasifier. It is believed that the surface of each droplet reaches the boiling temperature almost instantaneously; that the droplets then absorb heat from the hot gas; and that they vaporize over a finite time interval as they flow along with the gas stream. The resulting water vapor diffuses into and mixes with the main hot gas stream. At the same time, the ash particles lose heat to the now cooler gas that surrounds them. There is no direct transfer of heat between the liquid water and the ash since both are finely dispersed in the continuous gas phase and since both occupy only a very small portion of the total volume. All heat transfer takes place by way of the continuous gas phase which acts as an intermediary between the water and the ash. In a typical installation of the KTP, the liquid water spray and the ash occupy each only about 0.005 percent of the total volume.
The heat absorbed by the spray water may be broken down into two parts. The first part comprises: (1) the sensible heat of liquid water rising from its inlet temperature of about 115.degree.F (46.1.degree.C) to its boiling temperature at212.degree.F (100.degree.C); (2) its latent heat of vaporization; and (3) the heat to raise the temperature of the resulting steam from 212.degree.F (100.degree.C) to the final temperature at which the gas leaves thewaste heat boiler, which is about 350.degree.F (176.7.degree.C). The large amount of heat used to accomplish all of this is lost; leaving as sensible heat in the liquid streams from the scrubbers and from the gas cooler and being rejected to the atmosphere in the gas-cleaning cooling tower. the waste
The second part of the heat absorbed by the spray water heats the resultant steam from 350.degree.F (176.7.degree.C) to about 2060.degree.F (1115.5.degree.C), but this heat is recovered in the waste heat boiler.